From the release announcement:
This release addresses a handful of minor issues as well as a rather annoying problem where scheduled posts and pingbacks are not processed correctly due to incompatibilities with some hosts. If any of these issues affect you, give 2.9.1 a try. Download 2.9.1 or upgrade automatically from the Tools->Upgrade menu in your blog’s admin area.
WordPress 2.9.1 came less than a month after WordPress 2.9.